The use of liquid refractive elements in optical systems for the purpose of achieving color correction (i.e., correction of chromatic aberration) has been reported by only a very few investigators.
A technique was disclosed in U.S. Pat. No. 4,958,919, to R. D. Sigler for designing lens systems that are color-corrected at three or more wavelengths using liquid lens elements in combination with rigid (e.g., glass, plastic or crystal) lens elements. Particular examples of color-corrected lens systems using liquid lens elements were disclosed in U.S. Pat. No. 5,033,831, to R. D. Sigler, and in U.S. Pat. Nos. 4,911,538; 4,913,535; 4,915,483; 4,932,762 and 4,950,041, to P. N. Robb.
As discussed in U.S. Pat. No. 4,958,919, precise refractive index data at specified wavelengths are required for all the available liquids that are to be considered as candidates for use in forming liquid lens elements in color-corrected optical systems. However, there is a paucity of published refractive index data for most liquids.
In fabricating an optical system in which a liquid lens element is to be confined within a gap between two rigid (e.g., glass, plastic or crystal) lens elements, the rigid lens elements must be mounted so as to seal the liquid lens element within the gap. In a typical optical system (e.g., a telescope), rigid lens elements are mounted within a hollow cylindrical mounting device by means of an adhesive material, which is applied around the edges of the rigid lens elements to bond the edges of the rigid lens elements to interior surface portions of the mounting device. In order to confine a liquid lens element within a gap between two rigid lens elements, the adhesive material used to mount the rigid lens elements must be substantially impervious to the liquid comprising the liquid lens element. Adhesive materials ordinarily used for mounting refractive elements in optical systems have heretofore consisted of silicon rubber or epoxy resin.
It has been found experimentally from fabricating optical systems containing liquid refractive elements that adhesive bonds consisting of silicon rubber tend to swell when in contact with silicon oils, aldehydes, ketones, and to a lesser extent esters. In general, the swelling of a bond could cause the bond to fracture, and thus to lose effectiveness as a liquid-tight seal. Furthermore, the swelling of a bond securing a rigid lens element within a cylindrical mounting device produces edge forces on the rigid lens element. Such edge forces cause mechanical stresses in the rigid lens element that can introduce geometrical aberrations into the optical system.
It has also been experimentally observed that adhesive materials consisting of epoxy resin tend to undergo chemical decomposition when in contact with esters, aldehydes, ketones, aromatic hydrocarbons and aliphatic hydrocarbons. Thus, adhesive materials consisting of epoxy resin cannot be effectively used to form liquid-tight seals for confining many categories of liquids that, in terms of optical properties, would be good candidates for use as liquid refractive elements.
Practitioners in the relatively new art of fabricating color-corrected optical systems comprising liquid refractive elements have recognized a need for an adhesive sealant that is chemically non-reactive with silicon oils, esters, aldehydes, ketones, aromatic hydrocarbons and aliphatic hydrocarbons, and which does not swell upon contact with such liquids. More particularly, a need has been recognized for a sealant that can adhesively bond the edges of rigid (e.g., glass, plastic or crystal) refractive elements within a mounting device so as to confine a liquid refractive element consisting of a silicon oil, an ester, an aldehyde, a ketone, an aromatic hydrocarbon or an aliphatic hydrocarbon between the rigid refractive elements without leaking.